Nonwoven products of fabrics, comprise loosely assembled webs or masses of fibers bound together with an adhesive binder. Adequately bonded nonwoven fabrics have advantages over woven fabrics for a large variety of uses. It is known to form bonded nonwoven fabrics by impregnating, printing or otherwise depositing an adhesive bonding composition on a base web predominantly comprising relatively long fibers, including those of textile length of from about 1/2 inch (1.27 cm) to about 21/2 inch (6.35 cm), or more. These fibers may be of cellulosic or polymeric materials such as polyesters, polyamides, polyacrylates and the like. The base web of nonwoven fibers, to which the binder is applied, can be produced by carding, garnetting, airlaying, papermaking procedures, or other known operations. The operation of bonding fibers in place is much less expensive than conventional spinning and weaving. In comparison with woven fabric, the bonded nonwoven fabrics can be made in a much greater range of thicknesses per unit weight, with more homogeneous structures, no unravelling tendency, and with greater water absorbancy, porosity and resiliency, when required.
Representative of various binder compositions used in the art are U.S. Pat. Nos. 3,081,197; 3,137,589; and 3,380,851.
U.S. Pat. No. 3,081,197 discloses a nonwoven binder comprising interpolymers of vinyl acetate, another polymerizable compound as an internal plasticizer, and a post-curable comonomer such as N-methylol acrylamide.
U.S. Pat. No. 3,137,589 discloses binders comprising a copolymer of an alpha,beta-unsaturated carboxylic acid amide substituted on the nitrogen by at least one methylol group and another unsaturated polymerizable compound.
U.S. Pat. No. 3,380,851 discloses a binder comprising an interpolymer of vinyl acetate-ethylene-N-methylol acrylamide.
Such N-methylol acrylamide-type containing binder compositions are typically prepared by reacting a formaldehyde releasing material with a polymer containing acrylic acid amide units or by polymerizing the N-methylol acrylamide monomer with other ethylenically unsaturated monomers, the N-methylol acrylamide monomer having been formed by the reaction of a formaldehyde releasing material with acrylamide. In the manufacture of nonwoven binders, N-methylol acrylamide is typically used at a level of about 3-5%, based on total solids. These binder compositions afford objectionable quantities of free formaldehyde in nonwoven products such as disposable baby diapers. By "free formaldehyde" I mean that formaldehyde which is extractable with water, whether or not as a consequence of the following equilibrium reaction I which, for illustration, depicts a polymerized N-methylol acrylamide unit. ##STR1##
The chemical industry and, particularly, the disposable nonwovens market segment has become aware of the potential hazards of formaldehyde. Accordingly, the use of formaldehyde containing and formaldehyde releasing polymers has come under scrutiny.
There is a need for an N-methylol acrylamide containing binder for making nonwoven products containing low levels of free formaldehyde while maintaining acceptable wet and dry tensile properties.